Just a year after the Jaguars signed free-agent linebacker Clint Session to a five-year, $29 million contract with $11.5 million guaranteed, his career might be in jeopardy because of concussion problems.

Session has said he suffered three concussions last year, the last one on Nov. 20. He was placed on the injured-reserve list two days later.

He still hasn’t recovered and was the only player in camp not on the field Friday when the Jaguars started training camp. He was one of six players placed on the physically unable to perform list.

Coach Mike Mularkey declined to give a timetable for a return.

“We don’t know the future with him,” Mularkey said. “He is still having some problems with concussion symptoms, and we’re trying to make sure we do that correctly, especially with the concussions that are going on lately. We’re not pushing him or anything.”

The league is concerned about the concussion issue because more than 2,000 retired players are suing the league over the treatment they received for concussions.

“It’s been kind of an up-and-down process,” Mularkey said of Session’s recovery. “You get your hopes up, and maybe it will have something that will set him back. That has happened here as well.”

Session said that back in January he was having trouble resting, was getting two hours of sleep a night, couldn’t take naps and was having difficulty eating without becoming nauseous. He also was suffering headaches.

Session enrolled in a concussion treatment program at the University of Pittsburgh Medical Center at his alma mater. He said in April he had gone a month without symptoms but then apparently had a setback.

In Session’s absence, Russell Allen, who was signed as an undrafted free agent and worked his way up the depth chart to be the fourth linebacker, will become the starter.

PUP list

Besides Session, the other five players on the PUP list are defensive linemen Terrance Knighton and John Chick, guard Uche Nwaneri, tight end Zach Miller and offensive lineman Cameron Bradfield.

Although Session’s future is up in the air, Mularkey indicated he expects the other five to be ready to play fairly soon.

Chick is still recovering from a patella tendon injury he suffered last year, and Knighton is recovering from an eye injury he suffered in an incident at a club in April. Nwaneri, Miller and Bradfield suffered injuries recently while working out.

Roster moves

The Jaguars made two roster moves. They waived fullback Naufahu Tahi, who left the team for personal reasons and doesn’t plan to return. They signed tight end Gijon Robinson, who played with the Colts from 2008-2010 but hasn’t played since being cut by the Lions in the preseason last year. He gives them another tight end because Miller is sidelined.

Jones-Drew, Blackmon update

The team’s two holdouts, running back Maurice Jones-Drew and rookie receiver Justin Blackmon, missed their first day of practice Friday.

Jones-Drew wants a new deal, even though he has two years left on his contract. Blackmon isn’t allowed to report because he’s not signed.

Mularkey, who declined to say if he is fining Jones-Drew $30,000 a day, said Jones-Drew never called him to say he wouldn’t report.

Greg Jones, his blocking back, worked out with Jones-Drew in Miami last week and said, “I miss him.”

Jones added, “He’s a professional. He has a playbook. He knows it.”

Jones-Drew will have to learn the team’s new offense if he reports. Mularkey said the offense is “a friendly offense as far as learning,” but Mularkey said he will have to pick up some things in the protection scheme.

Mularkey said, “We’re not going to put him at risk of failure if he can’t pick it up fast enough. He’ll have to work a little harder on some things.”

On Blackmon, Mularkey said, “He’s got to catch up on what is being installed now and everything else he has missed. I believe this, if you unsure on what you’re doing, you can’t play to the speed you want to play at because you’re thinking too much.’’

Back to work

The Jaguars will practice twice again Saturday, with the 2:15 p.m. practice open to the public. They will have Sunday off.